1. Field of Application
The present invention relates to a control apparatus for an electric generator that is installed on a motor vehicle, for charging a battery of the vehicle and supplying power to electrical loads of the vehicle.
2. Description of Related Art
The electric generator of a motor vehicle (often referred to as the alternator of the vehicle) is a combination of an AC generator that is driven from the vehicle engine, and a rectifier apparatus, for producing a (unsmoothed) DC voltage which is supplied to charge the vehicle battery and to drive various electrical loads of the vehicle such as lamps, etc. Charging of the battery is performed such as to maintain the output voltage from the battery within a predetermined range. Methods are known for effecting this by controlling the level of output voltage from the electric generator (in general, by varying the level of field current of the generator) in accordance with the charge voltage of the battery (i.e., the output voltage that is produced by the battery when charging is halted, and whose value varies with the state of charge of the battery) or in accordance with the amount of power being consumed by the electrical loads of the vehicle, for example as described in Japanese patent first publication No. 6-54464 and Japanese patent first publication No. 2001-352795, referred to in the following as reference document 1 and reference document 2, respectively.
However with such a method of controlling the output voltage of the electric generator, the efficiency of operation of the electric generator may deteriorate as a result of variations in the running condition of the vehicle, the engine operating conditions, the state of charge of the battery and the degree of deterioration of the battery, etc. For example during engine idling, with the battery at a low level of charge, if the output voltage of the electric generator is increased in order to maintain the charge voltage of the battery at a predetermined value, then the operating efficiency of the electric generator will be lowered. Similarly, with such methods of controlling only the output voltage of the generator, it may not be possible to achieve a sufficiently high efficiency of recovering the dynamic energy of the vehicle motion during deceleration of the vehicle, by converting that energy to electrical power which is stored in the battery. Furthermore, since the electric generator is driven from the vehicle engine, a lowering of the operating efficiency of the generator will result in increased fuel consumption.
FIG. 7 is a map diagram of an example of electric generator efficiency characteristics for an engine-driven generator, in which the output current of the generator and the engine rotation speed are parameters. For example in the case of generation corresponding to the point “A” in FIG. 7, for the same value of engine rotation speed, if the output current of the electric generator is increased for example to point “B”, then the operating efficiency of the generator will be increased. Hence it can be understood that with methods of electric generator control which perform only control of the output voltage of the generator, it cannot be ensured that maximum operating efficiency for the generator can be achieved.
Furthermore with a conventional type of vehicle generator control system in which control is applied only to the generator output voltage, the system may attempt to maintain the output voltage of the generator at a high level while the engine is idling (i.e., with the vehicle halted). This will result in lowering the efficiency of operation of the electric generator, while in addition, due to fluctuations in the level of output current supplied from the generator, the amount of drive torque that must be applied to the generator by the engine will fluctuate accordingly. This causes irregularity of the engine idling speed (i.e., each increase in the level of drive torque results in a lowering of the engine speed of rotation), and such engine speed fluctuations during idling can impart an unpleasant sensation to the vehicle driver.